1. Field of the Invention
The present invention relates to a system and method for magnetic resonance (MR) imaging of heart wall motion in three dimensions using Spatial Modulation of Magnetization (SPAMM), and more particularly, to a system and method which permits direct assessment of three-dimensional heart wall motion from two-dimensional MR images.
2. Description of the Prior Art
As described in the afore-mentioned related patent applications, Spatial Modulation of Magnetization (SPAMM) creates multiple parallel "planes" or sheets of altered magnetization with easily controlled spacing and orientation. When used in conjunction with conventional magnetic resonance imaging (MRI), the intersection of these planes of altered magnetization with an image plane creates tagging stripes in the image that move with the underlying tissue in a similar manner as stripes created by selective saturation techniques do. Such selective saturation techniques are described by Zerhouni et al. in an article entitled "Human Heart: Tagging With MR Imaging--A Method For Noninvasive Assessment of Myocardial Motion", Radiology, Vol. 169, 1988, pp. 59-74. Also, when used in conjunction with cardiac-synchronized MR imaging of the heart with application of SPAMM at one phase of the cardiac cycle and imaging at a later phase, the stripe displacement in the image of the heart wall reflects the corresponding motion of the underlying wall between the times of SPAMM application and subsequent imaging.
SPAMM has been previously used to create two mutually orthogonal sets of planes of saturated magnetization, both orthogonal to the imaging plane, to produce a grid of tagging stripes in the image that can be used to track in-plane heart wall motion. However, in general, the motion of the heart includes a third dimension of motion, i.e., the motion through the imaging plane. One way to find the through-plane component of motion has been to acquire a second set of SPAMM tagged images which is oriented perpendicular to the first set. However, such a technique necessitates twice as much total image acquisition time and interpolation between the two image sets. A faster and simpler approach is desired for acquiring three-dimensional images of heart wall motion.
Accordingly, it is desired to develop alternative approaches to heart wall motion imaging with modified SPAMM sequences which can provide full three-dimensional motion information without the image acquisition and interpolation delays of existing techniques. Preferably, such approaches will allow for the provision of full three-dimensional motion information from a single set of two-dimensional tagged images so as to minimize acquisition and interpolation time. The present invention has been designed to meet this need.